A Queue ADT
A queue is a linear collection whose elements are added on one end and removed from the other. Therefore, we say that queue elements are processed in a first in, first out (FIFO) manner. Elements are removed from a queue in the same order in which they are placed on the queue.

public interface QueueADT
{
// Adds one element to the rear of the queue
public void enqueue (T element);

// Removes and returns the element at the front of the queue
public T dequeue();

// Returns without removing the element at the front of the queue
public T first();

// Returns true if the queue contains no elements
public boolean isEmpty();

// Returns the number of elements in the queue
public int size();

// Returns a string representation of the queue
public String toString();
}

Assignment: Using the QueueADT interface, implement the Queue ADT according to the API below:

■ Queue elements are processed in a FIFO manner—the first element in is the first element out.
■ A queue is a convenient collection for storing a repeating code key.
■ Simulations are often implemented using queues to represent waiting lines.
■ A linked implementation of a queue is facilitated by references to the first and last elements of the linked list.
■ The enqueue and dequeue operations work on opposite ends of the collec- tion.
■ Because queue operations modify both ends of the collection, fixing one end at index 0 requires that elements be shifted.
■ The shifting of elements in a noncircular array implementation creates an O(n) complexity.
■ Treating arrays as circular eliminates the need to shift elements in an array queue implementation.

Assignments:
1. Create a system using a stack and a queue to test whether a given string is a palindrome (i.e., the characters read the same forward or backward).

2. Create a system to simulate vehicles at an intersection. Assume that there is one lane going in each of four directions, with stoplights facing each direction. Vary the arrival average of vehicles in each direction and the frequency of the light changes to view the “behavior” of the intersection.